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  <div class="section" id="using-a-mutable-default-value-as-an-argument">
<h1>Using a mutable default value as an argument<a class="headerlink" href="#using-a-mutable-default-value-as-an-argument" title="Permalink to this headline">¶</a></h1>
<p>Passing mutable lists or dictionaries as default arguments to a function can have unforeseen consequences. Usually when a programmer uses a list or dictionary as the default argument to a function, the programmer wants the program to create a new list or dictionary every time that the function is called. However, this is not what Python does. The first time that the function is called, Python creates a persistent object for the list or dictionary. Every subsequent time the function is called, Python uses that same persistent object that was created from the first call to the function.</p>
<div class="section" id="anti-pattern">
<h2>Anti-pattern<a class="headerlink" href="#anti-pattern" title="Permalink to this headline">¶</a></h2>
<p>A programmer wrote the <code class="docutils literal notranslate"><span class="pre">append</span></code> function below under the assumption that the <code class="docutils literal notranslate"><span class="pre">append</span></code> function would return a new list every time that the function is called without the second argument. In reality this is not what happens. The first time that the function is called, Python creates a persistent list. Every subsequent call to <code class="docutils literal notranslate"><span class="pre">append</span></code> appends the value to that original list.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">append</span><span class="p">(</span><span class="n">number</span><span class="p">,</span> <span class="n">number_list</span><span class="o">=</span><span class="p">[]):</span>
    <span class="n">number_list</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">number</span><span class="p">)</span>
    <span class="nb">print</span><span class="p">(</span><span class="n">number_list</span><span class="p">)</span>
    <span class="k">return</span> <span class="n">number_list</span>

<span class="n">append</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="c1"># expecting: [5], actual: [5]</span>
<span class="n">append</span><span class="p">(</span><span class="mi">7</span><span class="p">)</span> <span class="c1"># expecting: [7], actual: [5, 7]</span>
<span class="n">append</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span> <span class="c1"># expecting: [2], actual: [5, 7, 2]</span>
</pre></div>
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<div class="section" id="best-practice">
<h2>Best practice<a class="headerlink" href="#best-practice" title="Permalink to this headline">¶</a></h2>
<div class="section" id="use-a-sentinel-value-to-denote-an-empty-list-or-dictionary">
<h3>Use a sentinel value to denote an empty list or dictionary<a class="headerlink" href="#use-a-sentinel-value-to-denote-an-empty-list-or-dictionary" title="Permalink to this headline">¶</a></h3>
<p>If, like the programmer who implemented the <code class="docutils literal notranslate"><span class="pre">append</span></code> function above, you want the function to return a new, empty list every time that the function is called, then you can use a <a class="reference external" href="http://en.wikipedia.org/wiki/Sentinel_value">sentinel value</a> to represent this use case, and then modify the body of the function to support this scenario. When the function receives the sentinel value, it knows that it is supposed to return a new list.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># the keyword None is the sentinel value representing empty list</span>
<span class="k">def</span> <span class="nf">append</span><span class="p">(</span><span class="n">number</span><span class="p">,</span> <span class="n">number_list</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
    <span class="k">if</span> <span class="n">number_list</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
        <span class="n">number_list</span> <span class="o">=</span> <span class="p">[]</span>
    <span class="n">number_list</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">number</span><span class="p">)</span>
    <span class="nb">print</span><span class="p">(</span><span class="n">number_list</span><span class="p">)</span>
    <span class="k">return</span> <span class="n">number_list</span>

<span class="n">append</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="c1"># expecting: [5], actual: [5]</span>
<span class="n">append</span><span class="p">(</span><span class="mi">7</span><span class="p">)</span> <span class="c1"># expecting: [7], actual: [7]</span>
<span class="n">append</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span> <span class="c1"># expecting: [2], actual: [2]</span>
</pre></div>
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<div class="section" id="references">
<h2>References<a class="headerlink" href="#references" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li><p>PyLint - W0102, dangerous-default-value</p></li>
<li><p><a class="reference external" href="http://stackoverflow.com/questions/101268/hidden-features-of-python#113198">Stack Overflow - Hidden Features of Python</a></p></li>
</ul>
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